Genetically Modified Brewer's Yeast
Overview Genetically modified brewer's yeast, including both ''Saccharomyces cerevisiae'' and ''Saccharomyces pastorianus'' species, is a recent advancement in brewing technology that affords higher resistances to alcohol content as well as more efficient fermentation of sugars. Although difficult due to many traits being polygenic, this can be achieved through selective breeding of yeast strains as well as manipulation of metabolic procees through natural or induced mutagenic methods. Genetic modification of yeast for brewing has typically been conducted with two specific goals in mind: to increase efficiency of the brewing process and to produce a higher quality end product.S. Saerins, C. Duong, and E. Nevoigt. (May, 2010). Genetic improvement of brewer's yeast: current state, perspectives and limits. Appl Microbiol Biotechnol. 86(5):1195-212 [http://www.ncbi.nlm.nih.gov/pubmed/20195857 PubMed] Uses, Advantages, and Disadvantages Genetically modified yeast affords advantages over normal yeast in that it can be used in brewing to make the overall process more successful and efficient. The main drawbacks are generally due to the fact that the desired phenotypes are polygenic and difficult to modify in a laboratory setting. This has given rise to effective methods of selective breeding and self-cloning of yeast, however the most striking results have been shown to be a result of direct genetic intervention via laboratory methods. Flocculation One such target of these modifications is the flocculation ability of the yeast. Flocculation is defined as, in simple terms, the ability of the yeast to aggregate as the fermentation process reaches its end. This is important for the brewing process as it makes the yeast simple to separate from the resultant beer product. A large amount of flocculation is desirable in brewing as weak flocculation can cause flavor issues due to fermentation issues as well as an increased difficulty in separation as the yeast does not rise to the top or sink to the bottom of the brew (depending on what type of brew is being made). Flocculation can be attributed to the FLO family of genes, which encode for the lectin-like family of proteins called flocculins. Flocculins cause floccculation by sticking out of the cellular membrane and binding to mannose present in the cell walls of other yeast cells, causing a tight but reversible binding.K. Verstrepen, G. Derdelinckx, H. Verachtert, F. Delvaux. (2003) Yeast flocculation: what brewers should know. Appl Microbiol Biotechnol. 61:197–205 [http://www.ncbi.nlm.nih.gov/pubmed/12698276 PubMed] Transcriptional control of flocculation has met with mixed success, however recently Govender and Co-workers (2008) were able to transcriptionally regulate floccullin production through the HSP30 and ADH2 mediated expression of FLO genes, leading to an increased level of flocculation. This was achieved through utilizing PCR and restriction enzyme ligation techniques to insert the specific primers and amplify DNA, which was then transformed into the desired yeast trains. Flocculation was assessed through chemical methods as well as stress-induced methods that simulate native ethanol production.P. Govender, J. Domingo, M. Bester, I Pretorius, and F. Bauer. (Oct. 2008) Controlled expression of the dominant flocculation genes FLO1, FLO5, and FLO11 in Saccharomyces cerevisiae. Appl Environ Microbiol.74(19):6041-52 [http://www.ncbi.nlm.nih.gov/pubmed/18708514 PubMed] Ethanol/Glucose Tolerance and Ethanol Production Another important target of yeast modification is its tolerance towards ethanol, one of the end products of the fermentation process and arguably the most important in the brewing process. This has been met with difficulty in the past as the genetic factors impacting ethanol tolerance of yeast strains have been found to not be monogenic. Through global transcription machinery engineering, Alper and Co-workers (2006) have found that the modification of the SPT15 gene, which modifies the spt15p transcriptional initiator, or the TAF25 gene , which codes for an associated protein. These were both carried out through error prone PCR amplification, screening for mutant strains, and selection mediated through growth in 5% ethanol containing media. The selected strains were found to lead to increased ethanol tolerance in comparison to wild type strains through differential expression of genes. This led to a phenotypic difference of both higher ethanol and higher glucose tolerance in the examined yeast cells. The mutant yeast was also found to have an ethanol yield increase of up to 15%. H. Alper, J. Moxley, E. Nevoigt, G.Fink, and G. Stephanopoulos. (Dec. 2006) Engineering Yeast Transcription Machinery for Improved Ethanol Tolerance and Production. Science. 2006 Dec 8;314(5805):1565-8.[http://www.ncbi.nlm.nih.gov/pubmed/17158319 PubMed] Hydrogen Sulfide Reduction and Sulfite Production Hydrogen sulfide is a common side product of yeast fermentation that leads to a taste quality issue in the beer and wine products that are commonly made using brewer's yeast. In relation, the presence of sulfite can actualy stabilize flavor, which leads to an interesting problem as both are part of the same pathway in beer production. One of the most effective methods to date for this process was employed by Hansen and Kielland-Brandt (1996) in which they inactivated the MET2 gene through constructing plasmids that disrupted expression of the gene and transformed it into their desired yeast strains. In theory, the silencing of this gene would disrupt expression of homoserine O-acetyl transferase, the enzyme responsible for converting hydrogen sulfide into homocysteine. Upon the removal of this enzyme, hydrogen sulfide accumulates and inactivates the sulfite reductase enzyme, which is responsible for reducing sulfite to hydrogen sulfide. This, in turn, then causes an increase in sulfite concentration.J. Hansen and M. Kielland-Brandt. (Sept. 1996) Inactivation of MET2 in brewer's yeast increases the level of sulfite in beer. J Biotechnol. 1996; 50(1):75-87. [http://www.ncbi.nlm.nih.gov/pubmed/8987848 PubMed] Another study by the same group inactivated the MET10 gene through similar methods which significantly reduced the yeast's ability to convert sulfite into hydrogen sulfide. These increases in sulfide are thought to stabilize beer and wine flavor and effectively negate any undesirable flavor that the end product may attain from the presence of increased hydrogen sulfide levels.J. Hansen and M. Kielland-Brandt. (Nov. 1996) Inactivation of MET10 in brewer's yeast specifically increases SO2 formation during beer production. Nat Biotechnol ;14(11):1587-91. [http://www.ncbi.nlm.nih.gov/pubmed/9634827 PubMed]S. Dequin. (Sept 2001)The potential of genetic engineering for improving brewing, wine-making and baking yeasts. Appl Microbiol Biotechnol;56(5-6):577-88. [http://www.ncbi.nlm.nih.gov/pubmed/11601604 PubMed] Restrictions and Terms of Use Currently, genetically modified yeast is approved for use all over the world in the production of foods. There is, however, a requirement to label such foods using these modified organisms if the current law in that area calls for it. The only exceptions to the labeling rule are if any liquid using yeast contains no yeast post-production or if genetically modified plants were used in the yeast cultivation solution.Yeast. GMO Compass Database. January 15, 2009. [http://www.gmo-compass.org/eng/database/ingredients/124.yeast.html Link] References